G. E. Cunningham
2/24/98

Mars Global Surveyor has just begun a short period (about 5 weeks) of time in which most of its science instruments have been turned off because there is simple not enough time in the ever decreasing orbit period to return data from science observations plus control the spacecraft and the orbital elements that are necessary to safely manage aerobraking. In the situation we are in today, we have suspended some science acquisitions until the Science Phasing Orbit begins around the first of April 1998, then resume full science acquisition with the full payload. This science acquisition will continue for about six months until the final phase (phase 2) of aerobraking begins. We are able to resume the science in the Science Phasing Orbit period because we will not be doing aerobraking and there will be no timing conflicts.

During the first phase of aerobraking in the Martian atmosphere, the Mars Global Surveyor spacecraft was able to acquire a sizable amount of science data. This large amount of data that was not planned for in the original MGS mission, but the opportunity for its acquisition was made available by the extended aerobraking period required with the unfortunate discovery of damage to one of the solar panel’s supporting structure. In the new mission plan, dictated by the slower aerobraking required by the damage, the spacecraft aerobrakes for about six months (aerobraking phase 1) until the orbital period has been reduced from 45 hours to 11.6 hours. Then it waits in this elliptical Science Phasing Orbit until Mars has moved around the sun to a point where aerobraking (aerobraking phase 2) can be resumed so that aerobraking will bring the orbit down into the 2 hour circular mapping condition at precisely the correct lighting conditions (2 PM ascending node). The mapping orbit conditions will be reached in January 1999 and mapping will actually begin in March 1999.

In its original mission plan, MGS would have arrived at Mars, been captured into orbit, several orbits of science observations would have been completed, and then the spacecraft would begin only a four month period of aerobraking designed to place the it into its desired mapping orbit shortly after the beginning of 1998.

The early science measurements conducted during the aerobraking phase 1 period provided contingency data sets, global context observations with unique lighting geometry and at minimum altitude within a season not available in mapping for two earth years. Also provided was a chance for early instrument evaluation, product production testing and preparations for the mapping operation level of activity. Many more observations will be conducted during the Science Phasing Orbit.

Because the damage to the solar array’s supporting structure that requires that we aerobrake more slowly, we have been able to continue the science acquisition process much longer than we could have supported in the original mission plan. Even in the original plan, there was a point at which the science instruments would have been turned off because in that orbit there would be a shortage of electrical power for the instruments. A shortage of electrical power is not now a limitation in the orbit that we are in.

During this whole period while we get the spacecraft into the correct mapping orbit, our first priority in conducting spacecraft operations is to assure that we will achieve the mapping orbit in order to meet the mission’s objectives (remember MGS’s objective is to conduct a full Martian year of systematic mapping of the Martian surface and atmosphere). Second priority is to assure the continued health of the science payload to assure its proper operation in the mapping orbit. Thus, the in-flight events that support these priorities are the ones that we will execute. The first priority events include: the aerobraking passes themselves, which includes turning the spacecraft from its earth pointed attitude to the aerobraking attitude and proper positioning of the solar panels as drag surfaces; the playback of engineering data from the spacecraft that characterize how the aerobraking worked; acquisition of radio navigation data to help manage the aerobraking process; periods to load the sequences of commands that are necessary to control the aerobraking passes and the propulsive maneuvers used to fine tune the aerobraking altitude; and finally the aerobraking propulsive control maneuvers themselves. The second priority events include special attitude changes of the spacecraft twice each orbit necessary to keep the laser altimeter instrument warm enough.

What we have found, and what has caused us to have to stop taking some science data, is that there is not enough time in the orbit after it got below a period of 16 hours to accomplish all of these high priority activities and have time to playback science data also. So, in order to preserve life time of the Mars Orbiter Camera and the Thermal Emission Spectrometer instruments, they were turned off. Radio science data will still be acquired every time the spacecraft is occluded by the planet.

At this time, when the spacecraft takes science data, its does so only during specific portions of the orbit that are dictated by the orbital geometry, the spacecraft’s attitude, and the instruments’ capabilities. For instance, the magnetometer can acquire data all around the orbit, but its best data is when the spacecraft is closest to the planet. The electron reflectometer cannot take data when the spacecraft is in the top of the atmosphere because its high voltage has to be off then to protect it from arcing. The thermal emission spectrometer can take data all around the orbit because it has a mirror that sweeps its field of view across the planet many times each orbit. The laser altimeter is off because it has to be below an altitude of 700 km to take data and the spacecraft is below this altitude for such a little time each orbit that is not prudent to waste laser shots on such short opportunities. The camera is able to take data only when it is pointed at the planet, and for most of the orbit it is not pointed at the planet so that the spacecraft’s high gain antenna, which is fixed to the side of the spacecraft now 90 degrees from the pointing direction of the camera, can communicate with Earth. Just after the spacecraft does its aerobraking drag pass, and it is turning back to the attitude that points the high gain antenna toward the earth, the camera has a few minutes when its field of view is swept across the planet and it can take a few meaningful pictures.

MGS is designed for its mapping mission orientation, when the science instruments are pointed straight down at the planet’s surface. During mapping, which starts in March 1999, the spacecraft will take science data all around the orbit because the high gain antenna will be deployed and able to track the Earth, and the solar panels can track the sun while the instruments are always pointed directly down at the Martian surface. The instruments are fixed to the spacecraft structure. They are not on an articulating platform that would make them easy to point.

But, while the spacecraft is the aerobraking periods, the ability to point the camera at the surface is limited by the competing attitude requirements of the aerobraking drag configuration, keeping the high gain antenna pointed at the earth, and keeping the solar panels in the sun light.

During the Science Phasing Orbit period, when there is no requirement for the aerobraking drag orientation, the spacecraft can be turned to point the camera and the laser altimeter at the surface during the closest point the surface while sacrificing communications to Earth for a short time. Since there are also no requirements to send commands to the spacecraft to manage the aerobraking and orbit altitude, time is now available to play the science data to Earth, even though the orbit period will be only 11.6 hours long.

When the Science Phasing Orbit period begins in late March, all the science instruments will be powered on, and they will remain on for six months until we are ready to start the final four months of aerobraking (aerobraking phase 2).